Pulsed ferromagnetic microwave generator



May 17, 1966 M. w. MULLER 3,252,111

PULSED FERROMAGNETIC MICROWAVE GENERATOR Filed April 24, 1962 M HINVENTOR. MARCEL w. MULLER X x *Qm ATTORNEY United States Patent Olilice3,252,111 Patented May 17, 1966 fornia Filed Apr. 24, 1962, Ser. No.189,804

4 Claims. (Cl. 331-96) The present invention relates to the generationof microwave frequency power, and more particularly to a novel microwaveradiation technique which utilizes the interaction of a pulsed magneticfield with a ferromagnetic crystal exhibiting metastable magnetization.

It has been realized for several years that if the magnetization of asaturated ferromagnet can be brought into a state in which it istransiently unaligned with respect to an external static (D.C.) magneticfield, the magnetization can then precess about this field and radiatemicrowave pulse power at the precession frequency. Further, it has beenrealized that such a device would have certain potential advantages,including a high power output due to the large number of electron spinwhich contribute to the magnetization even at room temperature.

A summary of prior devices which have been utilized or proposed for thispurpose is given in the following papers by B. J. Elliot et al.: PulsedFerrimagnetic Microwave Generator, Journal of Applied Physics, vol. 31,pp. 4008-4018 (May- 1960); Pulsed Millimeter-Wave Generation UsingFerrites, I.R.E. Trans, vol. MTT-9, pp. 9294 (Jan. 1961). In general,the power generation schemes utilized in these prior devices requireeither impractically short rise times in the pulsing of the DC. field,or the addition of a microwave power supply.

It is the principal object of the present invention to provide a novelpulsed ferromagnetic microwave generator which utilizes pulsed D.C.fields of practical'rise times without requiring microwave excitation.Generally speaking, this is accomplished by putting a ferromagneticsample into a state of metastable magnetization and then removing themetastable energy minimum of the sample whereby the magnetizationprocesses about the direction of absolute energy minimum.

Various features and advantages 'of the present invention will becomeapparent upon a perusal of the following specification taken inconnection with the accompanying drawing wherein:

FIG. 1 is a schematic representation of a generator in accordance withthe present invention, the microwave structure being shown in isometricview with the exterior walls broken to expose the interior details, and

FIGS. 2a, 2b, 2c, and 2e are polar energy diagrams for explaining theinteraction of the sample magnetization and applied field during thevarious pulsing intervals a through e in the generator of FIG. 1.

The samples of interest in'the present invention include those in whichthe uniform magnetization of a saturated monocrystalline ellipsoid canassume twoor more orientations. The energy of the sample is lowest inone of these orientations; the others are metastable, corresponding tolocal but not absolute minima of the energy surface. Such a sample is'said to exhibit metastable magnetization. As is well known, such samplesmay be obtained, for example, from several known compounds of both cubicand hexagonal structure with suitable crys talline anisotropy constants.

Referring to FIG. 1, we consider, for purposes of illustration, a thindisc 1 of cubic material with positive anisotropy out parallel to a(100) plane and thus containthe resultant field H established during thevarious pulsing four directions (100) of easy magnetization. The sampleis mounted on the end wall of a rectangular cavity resonator 2 with oneof these'easy directions oriented in the direction x perpendicular tothe narrow walls of the resonator.

Disposed about the sample 1 and supplying magnetic fields thereto arecoils 3, 4 and 5, indicated in an exploded and schematic manner. Thecoils 3 and 4 are disposed respectively along the mutually perpendicularx and z axes; and the coil 5 is disposed along axis w which makes anangle of 135 with the z axis. The DC. pulse source 6 represents any wellknown circuit arrangement for supplying current pulses to the coils 3,4, S which are of the general form indicated above the lines 3', 4, 5coupling the source 6 to the coils.

FIG. 2 illustrates the interaction of the sample 1 with ing intervals athrough 2.

During the interval a, a field is established in the x direction whichis of sufficient magnitude to produce a saturation magnetization M ofthe sample. The dotted line is a polar plot of the free energy of theanisotropic sample crystal in the absence of an external field, thisplot displaying energy minima along the x and z axes and an energymaximum at an angle 0 (measured from the z axis) of 45. Since thesaturating field H is larger than the anisotropy field H,, of thecrystal, the net energy plot of the sample, shown by a solid curve,exhibits a single energy minimum or which is in the direction (x) of theapplied field.

During the interval b, the applied field H is reduced to a value in therange AH,, H H,, whereby these fields interact to produce a net energyplot wherein a local energy minimum 5, due to the anisotropy field,appears in the z direction.

During the interval 0, an increasing field is applied in the z directionvia coil :set 4 (and the x direction field is preferably decreased)whereby the result-ant applied field H rotates towards the z axis. Whenthe angle 0 of the H vector is less than 41, the energy minimum 3 nearthe z axis becomes deeper than the energy minimum or near the x axis.However, the magnetization vector M is constrained to the less deepminimum on since there is an intervening energy maximum (potentialbarrier). This is a condition of metastable magnetization.

During interval d, the resultant field H, and its tendency to set up anenergy minimum, continues to rotate toward the z axis until, at theonset of interval e, it is substantially in the z direction. The localminimum a disappears and the magnetization M is left in an unstableposition at an angle to the applied field H.

During the interval 2, the in gnetization M undergoes precession aboutthe new equilibrium z direction, thereby radiating .a pulse oi energyinto the surrounding cavity resonator 2 at the precession frequency. Thecavity resonator is tuned for resonance at the radiation frequency in adominant TE mode, the sample 1 being located in a position of maximummagnetic field and minimum electric field for such a mode. The power sogenerated is coupled via iris plate 7 and connecting waveguide 8 to anexternal load, for example an antenna.

Taking dilute (approximately 20%) cobalt ferrite as an example, theinstability of interval e sets in when 0 is For sufficien-tly thinsamples N =transverse demagnetization factor N =longitudinaldemagnetization factor a pulse having a rise time on the order of 20nanoseconds is sufiicient, which is a much less stringent condition thanthat presented by prior D.C. pulse schemes. In particular, it is to benoted that the effective precession frequency just prior to pulsingapproaches zero, so thatone is not troubled with the requirement thatthe field is to be pulsed in a time short compared to a precessioncycle. As a further advantage, it should be noted that the magnetizationmay be initially placed at a substantial angle ratio less than about .01

with reference to the field about which it preceses, 48.7

in the present example, Whereas prior schemes using microwave excitationare limited to angles on the order of 1 with a corresponding limitationon the pulse power generated.

constants K and K and the magnetization M. For a (100) disc with N ==0.land N =0.8 cut from a material with 41rM=5000 gauss, first orderanisotropy field H =2000 oersteds, and an electron spin g-factor of 2.3,the radiated frequency varies from 8 to 12 lime/sec. as the appliedfield H is changed from 1700 to 1100 oersteds. The energy that ispotentially available for the radiation is the difference between themetastable and final energy mini-ma which, for this example, varies fromx10 to ergs per cm. over this frequency range. For a 0.01 cm. sample, a1 sec. pulse, and an efficiency of 10%, this would furnish a pulse powerof 1 to 5 Watts.

The cobalt ferrite example is characterized by a rather largeferromagnetic resonance linewidth and hence a correspondingly short freeprecession time during which pulse power may be obtained. For longerpulses a material such as yttrium iron garnet, which exhibits ananomalous anisotropy at liquid helium temperatures, may be found useful.

Since many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A device for generating high frequency power comprising: ananisotropic fer omagnetic disk-like sample capable of exhibitingmetastable magnetization; a cavity resonator having an end wall, saidsample being mounted on such wall within said resonator at a location ofmaximum magnetic field for the resonant mode of said resonator at theprecession frequency of said sample; first, second and third coil meansrespectively coupled to said sample, and disposed radially thereto, saidfirst and second coil means being substantially perpendicular to eachother; and pulsing current means coupled to said first coil and adaptedfor first magnetizing said sample for generating a magnetic field alonga first axis defined by said first coil means, and for then reducing thecurrent to said first coil to decrease the magnitude of such field, saidpulsing current means coupled to said second coil means and adapted forincreasing the current thereto so that such field rotate-s toward anaxis intermediate the axes defined by said firs-t and second coil meansand for establishing a condition of metastable magnetization; saidpulsing current means coupled to said third coil means for applying apulse thereto so that such field rotates towards the axis defined bysaid second coil means to develop a condition of unstable magnetization,whereby pulsed energy is radiated in said cavity resonator. V 2. Adevice according to claim 1 wherein said sample is a single crystal inthe form of a disc sufliciently thin that the ratio of the transversedema-gnetizing factor to the longitudinal dem agnetizin-g factor is lessthan .01.

3. A solid-state microwave oscillator comprising: an anisotropicferromagnetic material, a cavity resonator having an end wall, saidmaterial being mounted on such wall within said resonator at a locationof maximum magnetic field for the resonant mode of said resonator at-magnetic field away from such first axis and towards a directionintermediate the first axis and a second axis defined by said secondcoil means, said first and second 'axes being substantially orthogonal,and third coil means 'for applying a magnetic field pulse substantiallyat right angles to such intermediate direction so that a condition ofunstable magnetization is produced.

4. Apparatus (for generating high frequency power comprising: acrystalline anisotropic material having directions of easy magnetizationin at least first and second directions; a cavity resonator having anend well, said material being mounted on such wall Within said resonatorat a location of maximum magnetic field that is established by tuningsaid resonator to a resonant mode at the frequency of precession of saidmaterial; and means for pulsing said material, including first, secondand third coil means, said first and second coil means disposedorthogonally, said first coil means serving to establish a magneticfield, said second coil means serving to establish a condition ofmetastable magnetization, said third coil means serving to rotate themagnetic field by pulsing so that a condition of unstable magnetizationarises, whereby pulsed energy is radiated in said cavity resonator.

References Cited by the Examiner UNITED STATES PATENTS 2,873,370 2/1959Pound 33 1---107 3,087,122 4/ 196 3 Rowen 33l-94 3,164,768 1/1965Stiglitz e't al. 3304.8 X 3,165,711 1/1965 Drurn hcller etal. 333-l.l

ROY LAKE, Primary Examiner.

4. APPARATUS FOR GENERATING HIGH FREQUENCY POWER COMPRISING: ACRYSTALLINE ANISOTROPIC MATERIAL HAVING DIRECTIONS OF EASY MAGNETIZATIONIN AT LEAST FIRST AND SECOND DIRECTION; A CAVITY RESONATOR HAVING AN ENDWALL, SAID MATERIAL BEING MOUNTED ON SUCH WALL WITHIN SAID RESONATOR ATA LOCATION OF MAXIMUM MAGNETIC FIELD THAT IS ESTABLISHED BY TUNING SAIDRESONATOR TO A RESONANT MODE AT THE FREQUENCY OF PRECESSION OF SAIDMATERIAL; AND MEANS FOR PULSING SAID MATERIAL, INCLUDING FIRST, SECONDAND THIRD COIL MEANS, SAID FIRST AND SECOND COIL MEANS DISPOSEDORTHOGONALLY, SAID FIRST COIL MEANS SERVING TO ESTABLISH A MAG-